1. Field of the invention
The invention relates to transmission of signals in a borehole, and more particularly to transmission of acoustical signals through a drill pipe.
2. Technical considerations
The desirability of telemetering information to the surface from a borehole while drilling has long been recognized. The best method presently in use is to cease drilling and lower an electronic instrument package into the borehole by means of a conductor cable to measure temperature, pressure, inclination, direction, etc. Borehole conditions of interest are measured and transmitted electrically up the cable to the surface where they are interpreted and displayed on surface instruments. After use the instrument and cable must be removed from the borehole before recommencing drilling in rotary drilling is used. Use can be left in place until another section of drill pipe must be added to the drill string, however, if a downhole mud motor is used to drive the drill bit. Insertion and removal of such instruments require a considerable amount of time during which drilling cannot occur. It has been estimated that elimination of such costly drilling rig down-time by means of while-drilling telemetry systems could eliminate 5% to 6% of direct production platform drilling costs in offshore platforms.
The applicant has disclosed in previous patents, e.g., U.S. Pat. No. 4,019,148, utilization of an acoustical transmission system in which an acoustical signal is inserted into a drill string at one location at a "nominal" frequency and is detected at a second location. The signal is then repeated and retransmitted at a second nominal frequency to a second detector, where it is in turn repeated and retransmitted to a third detector located at a third position in the drill string. After the third repeater the sequence of frequencies is repeated in subsequent repeaters until the signal reaches the surface and is detected and read out. It was disclosed in U.S. application Ser. No. 644,686 now U.S. Pat. No. 4,019,148 that these nominal frequencies are in fact two frequencies that are separated by only a very small frequency difference (e.g., 20 Hz.). In that application it was disclosed that information is encoded into an intelligible form for acoustical transmission along the drill string into binary coded data according to the frequency-shift-keyed modulation (FSK) system. The information concerning borehole parameters is converted from analog or other form to digitally coded words which are used to modulate the FSK system. The FSK system represents digital data by shifting between the aforementioned two nominal frequencies. One frequency is used to represent a binary "zero" and the other to represent a binary "one," and by shifting between the two frequencies in the proper sequence binary words can be represented. The encoded FSK signals can then be used to drive an electro-acoustical transducer, or other suitable device, which induces the desired signals into the drill string in the form of acoustical signals.
It has been found that several problems are associated with this type of modulation system. It was found to be a characteristic of drill pipe that signals once induced tend to continue to oscillate or "ring" long after the driving signal has been removed. This is a fact that was not recognized previous to the present invention by either the applicant or by others. It was assumed that drill pipe would act like other acoustical conductors and would dampen out any ringing by the well known process of attenuation. It has been discovered, however, by the applicant that for unknown reasons, whether it be the tubular shape of the drill pipe, the length of the drill pipe, the manner in which drill pipes are conventionally interconnected, or other reasons, the assumptions extant in the prior art are erroneous. It was also found that the problems are compounded by the use of two frequencies that are close together. Phase delays and ringing found by the applicant to be inherent in the transmission of acoustical signals in a drill pipe cause interference and intermodulation between the two different signals, thereby destroying the coherency and thus the informational value of the signals.